DIY Spot Welder Using Two Microwave Power Transformers — a cautious how-to

⚠️ Important disclaimer (please read)

Disclaimer: This DIY involves very high-voltage transformers and wiring and could fatally electrocute you or someone else, start fires, or cause serious injury or property damage. This project is for entertainment/educational purposes only and does not endorse anyone attempting it. Do it at your own risk. If you are not experienced with mains electricity and transformer safety, stop now and buy a commercial spot welder or ask an experienced professional.

What this blog will (and won’t) do

What you’ll get here:

• A conceptual explanation of how to use two microwave power transformers (MOTs) to create a DIY spot welder.

• A general materials and tools list.

• Safety guidance and recommended protective measures.

• Alternatives that are much safer and often cheaper in the long run.

What I will not provide:

• Exact wiring diagrams or step-by-step instructions for modifying MOTs or wiring mains terminals in a way that could cause electrocution.

• Specific measurements, voltages, or instructions to bypass safety features on appliances.

• Anything that would knowingly enable unsafe modification of high-voltage equipment.

The working spot welder:

How to beat the Sprinter blues "AdBlue"

There are many confusing info circulating the net about DEF fluids, brands to use, etc. Some Sprinter owners have been told by MB dealers to only use an MB branded DEF fluid "AdBlue" on their Sprinters or the DEF system could malfunction.

The truth is, if the DEF fluid is ISO-22241 certified then it is ok to use as per Sprinter operating manual. I've been using "BlueDEF" brand from my local auto parts store for many years without any problems. 

When my Sprinter NOx sensor went south a few years ago, MB dealer even tested the "BlueDEF" branded fluids in my tank and it passed to their standards, no surprise there. I'm not claiming I'm a DEF expert, just a user trying to save a buck from overpriced MB pee (ok, it's not actually a pee but rather it's a combination of 67.5% deionized water and 32.5% urea.)

Safety Issue: Continental Spare Tire Carrier Failure

I feel compelled to share this. One morning before heading to work, I decided to check on my spare tire and noticed that it was loose as if it was unbolted from the carrier. I immediately removed the plastic tire cover and to my demise, the flat metal bracket where the tire is bolted on has completely broken off and the tire was just hanging by its rim on top of the remaining broken bracket.  The failure was at the 90-degree bend of the bracket and to the weld joint where it meets the vertical square tubing (see picture). The welded spot has probably failed first as evident by some rust stains along the weld joints, then from shear load of the tire and road vibrations the bracket just could not handle the stress and broke off at the weak point. The welding looks sloppy with bad metal penetration and to make it worst, it was only welded to one side of the bracket. If I have driven the RV that day without noticing it, the tire could have simply slid out and have fallen on the road, which could cause a vehicle crash. I took the broken tire bracket to a local weld shop and have gussets added to corners for more suport.

For anyone with the “Continental Tire Carrier” option, it would be wise to thoroughly inspect it for signs of failure.